Solution NMR Spectroscopy with Isotope-Labeled Cysteine (¹³C and ¹⁵N) Reveals the Surface Structure of l-Cysteine-Coated Ultrasmall Gold Nanoparticles (1.8 nm)

Abstract

Ultrasmall gold nanoparticles with a diameter of 1.8 nm were synthesized by reduction of tetrachloroauric acid with sodium borohydride in the presence of l-cysteine, with natural isotope abundance as well as 13C-labeled and 15N-labeled. The particle diameter was determined by high-resolution transmission electron microscopy and differential centrifugal sedimentation. X-ray photoelectron spectroscopy confirmed the presence of metallic gold with only a few percent of oxidized Au­(+I) species. The surface structure and the coordination environment of the cysteine ligands on the ultrasmall gold nanoparticles were studied by a variety of homo- and heteronuclear NMR spectroscopic techniques including 1H–13C-heteronuclear single-quantum coherence and 13C–13C-INADEQUATE. Further information on the binding situation (including the absence of residual or detached l-cysteine in the solution) and on the nanoparticle diameter (indicating the well-dispersed state) was obtained by diffusion-ordered spectroscopy (1H-, 13C-, and 1H-13C-DOSY). Three coordination environments of l-cysteine on the gold surface were identified that were ascribed to different crystallographic sites, supported by geometric considerations of the nanoparticle ultrastructure. The particle size data and the NMR-spectroscopic analysis gave a particle composition of about Au174(cysteine)67.

Controlling the Surface Functionalization of Ultrasmall Gold Nanoparticles by Sequence‐Defined Macromolecules

“…Interestingly, the addition of the PEG group did not have a significant effect on the number of ligands per nanoparticle, thus further highlighting the importance of the OAA segment to control the nanoparticle functionalization. The footprint of a cysteine molecule on 1.8 nm Au ca.174 (cysteine) ca.67 nanoparticles was about 0.15 nm 2 [5b] . This is about two times the value of OAAs, indicating a denser packing of the OAAs on the nanoparticle surface, possibly induced by non‐covalent interactions between the ligands.…”

“…Figure 3 shows representative 1 H NMR spectra of dissolved short‐2 and of nanoparticle‐conjugated short‐2. There was a considerable broadening of the 1 H‐resonances after binding of short‐2 to the nanoparticle surface due to longer relaxation times and the presence of the metallic core [5b, c, 8a, b] . Another peculiarity was the disappearance of the peaks of the cysteine building blocks after binding to the particle surface due to the vicinity to the metal surface.…”

“…Density functional theory (DFT) calculations have shown that Au‐S bonding arises from the hybridization of p‐like S orbitals with d‐like Au orbitals [25] . Ultrasmall nanoparticles with fcc structure can show different shapes like tetrahedra, cuboctahedra, or truncated octahedra [5b, 26] . DFT calculations have also shown that the adsorption energy of sulfur on the surface of different fcc metals like Au, Ag, Cu and Pt follows the general trend (100) ≈ (110) > (111) [27] .…”

“…They are typically a bit larger than atom‐sharp gold clusters (1 to 2 nm) and have a diameter between 1 and 3 nm, although there is no strict size range for them which is generally accepted [7] . The fact that NMR spectroscopy is possible with such ultrasmall nanoparticles is advantageous for a quantitative assessment of their surface composition, especially to determine the number of ligands and thus the degree of surface functionalization [5b, c, 8] …”

Ultrasmall gold and silver/gold nanoparticles (2 nm) as autofluorescent labels for poly(D,L-lactide-co-glycolide) nanoparticles (140 nm)

“…Ultrasmall gold nanoparticles (diameter 1-3 nm) which are meeting the size range of atomically sharp gold clusters [40][41][42][43] often show a high fluorescence in the visible part of the spectrum that has been well documented [33,[44][45][46]. To stabilise these ultrasmall nanoparticles, thiols, dendrimers or thiol-based polymers can be used [28,[47][48][49][50][51]. Particles stabilised with small molecules carrying thiol groups show fluorescence with low quantum yield (<1%) [47].…”